In recent years, it has become possible to easily obtain a coherent light by use of a laser, and commercial use of holograms has also become considerably widespread. Particularly, for exchange tickets for money and credit cards, from an anticounterfeit standpoint, it has been generalized to create a hologram at a part of the medium.
Currently, most of the commercially used holograms are ones created by recording an original image on a medium as interference fringes by an optical method. That is, in the optical method employed, an object from which an original image is formed is prepared, light from the object and reference light are led onto a recording surface applied with a photosensitive material by use of an optical system such as a lens, and interference fringes are formed on the recording surface. Although this optical method requires an optical system with a considerably high degree of accuracy for obtaining a clear reproduced image, it is the most direct method for obtaining a hologram and is therefore the most industrially widespread method.
On the other hand, a method for creating a hologram by forming interference fringes on a recording surface by a computer-aided operation has also been known. A hologram created by such a method is generally called a “Computer Generated Hologram (CGH)” or simply a “computer hologram.” The computer hologram can be obtained by simulating, on a computer, an optical interference fringe generation process, in a manner of speaking, and processes for generating an interference fringe pattern are all carried out as an operation on a computer. After image data on the interference fringe pattern has been obtained by such an operation, physical interference fringes are formed on an actual medium based on the image data. Concretely, for example, the following method has been put into practical use. That is, image data on an interference fringe pattern created by a computer is given to an electron beam drawing apparatus and electron beams are scanned on a medium so as to form physical interference fringes. For example, Japanese Unexamined Patent Publication No. 2000-214750 has disclosed a creation method of a computer hologram that is capable of reproducing an original image with grayscales as faithfully as possible.
As described above, computer holograms are a field for which great demand is expected in the future, however, at this time, there are some problems to be solved in realizing commercial use. One of the great challenges is to enable reproduction of a high-quality grayscale image. A computer hologram created by a conventional method has a problem in that reproduction quality of a grayscale image declines in comparison with an optical hologram.
For example, when the method disclosed in Japanese Unexamined Patent Publication No. 2000-214750 cited above is used, it becomes possible to reproduce an original image with grayscales at a certain level of quality. However, a phenomenon that unnecessary noise components are overlapped is recognized in a reproduced image, and it is undeniable that, in comparison with a hologram created by an optical method, reproduction quality of a grayscale image declines. Particularly, in the lighting environment of the real world where a multiple of light sources exist, a reproduced image is observed in a condition mixed with a large number of noise components, and visibility cannot but decline in comparison with an optical hologram.
It is therefore an object of the present invention to provide a computer hologram that is capable of obtaining a reproduced image as clear as possible by reducing unnecessary noise components generated during observation as much as possible.